A phenolic resin which is a thermosetting resin is mainly and widely used as a binder that bonds materials which become base materials of a molded product. Since the phenolic resin is excellent in mechanical characteristics, electrical characteristics, or adherence, it is used in various fields. Particularly, in recent years, the amount used of a friction material in which the phenolic resin is used as the binder in vehicles, railway vehicles, or the like has increased.
Among these, a liquid resol-type phenolic resin is generally used in a friction material which is called a wet paper friction material and is used in an automatic change gear or the like of an automatic vehicle or the like. The required characteristics with respect to phenolic resin for the wet paper friction material have increased year by year. Particularly, in order to improve the friction characteristics, demand for improving flexibility of the phenolic resin has increased. However, a general cured material of a phenolic resin has a property of being excellent in mechanical properties, but since it is firm and fragile, it cannot be said that the general cured material thereof is excellent in flexibility.
As a method of solving the above-described problem, an attempt of improving the flexibility using drying oil or the like has been examined as a modifier in a reaction while synthesizing the phenolic resin (for example, Patent Document 1). Such a drying oil-modified phenolic resin to which a flexible aliphatic hydrocarbon group is introduced has a characteristic of high flexibility compared to an unmodified phenolic resin.